1. Field of the Invention
The present invention relates to a thread spreading apparatus for use in an overlock sewing machine. More particularly, the present invention relates to a thread spreading apparatus for use in an overlock sewing machine which is capable of performing over-edge chain stitching and other stitching (e.g., special stitching such as multi-thread chain stitching or a covering chain stitching).
2. Description of the Related Art
There has heretofore been known an overlock sewing machine which is designed to perform multi-thread chain stitching or covering chain stitching by using two vertically moving needles and a looper which oscillates both longitudinally and laterally. FIG. 32 in the accompanying drawings shows a seam of multi-thread chain stitches formed with two needles and three threads as in such an overlock sewing machine. As illustrated in the figure, two needle threads T1 and T2 extend in parallel, and a looper thread R is used for crossover stitching between the two needle threads T1 and T2.
In multi-thread chain stitching or covering chain stitching, each needle must descend into a triangle defined by a needle thread, a looper thread, and a looper. As a thread selvedge device used for this purpose, a device which forms the above-described triangle by retaining the looper thread is known as disclosed in Japanese Patent Public Disclosure (KOKAI) No. 52-12058 (1977).
However, in the case of an overlock sewing machine which is capable of over-edge chain stitching and special stitching, e.g., multi-thread chain stitching or covering chain stitching, if a lower looper for special stitching as described above is provided in addition to a pair of upper and lower loopers for over-edge chain stitching, the looper mechanism becomes complicated. Therefore, it is demanded to perform the above-described special stitching by using the lower looper for over-edge chain stitching.
In such a case, as shown in FIG. 33(A), when needles 8 and 10 descend, a lower looper 14 moves leftward (in the direction of the arrow), and hence needle threads T slide rightward relative to the lower looper 14. Therefore, each of the needles 8 and 10 undesirably drops on the left-hand side of the corresponding needle thread T and thus fails to enter the triangle defined by the needle thread T, the looper thread R and the looper 14. FIG. 33(B) shows a state where a proper triangle is formed and the needles 8 and 19 can descend into the triangle. It is necessary in order to form such a proper triangle to retain the needle threads T, not the looper thread R as in the above-described conventional technique.